1. Field of the Invention
The invention in general relates to electroacoustic transducers, and particularly to a technique for eliminating unwanted mode resonances.
2. Description of the Prior Art
A piezoelectric transducer will provide an acoustic signal in response to excitation by electrical energy. The frequency of the output signal will be a function of various parameters, one of which is the dimensions of the body of transducer material. For example, there are many applications for plate or disc transducers having a certain thickness and a certain transverse dimension, which for the case of a disc, will be its diameter.
For some applications the disc is shock excited with a step voltage waveform to transmit an acoustic pulse to receiving apparatus. The problem arises, however, in such operation in that mode interaction causes unwanted acoustic energy in the bandwidth of interest. For example, as the ratio of the thickness of the disc to its diameter increases there results an unwanted radial mode resonance responding to the shock excitation to the same degree as the desired thickness mode resonance. This interaction can at times be so severe as to significantly reduce the accuracy of the system in which the transducer is utilized.
In order to eliminate unwanted modes of oscillation, several prior art tranducers utilize piezoelectric elements having a first series of grooves cut into a major surface thereof with a second series of grooves cut into the same surface, at right angles to the first series of grooves so as to form in the major surface a two-dimensional array of posts.
With such structures, electrical contact may be made to the flat surface of the piezoelectric element with relative ease, however the adequate attachment of electrode means to all of the upstanding posts may present a difficult and time-consuming problem. In addition, by the nature of the cuts in the piezoelectric element, an unsymmetrical thermal expansion may be experienced which will bend or warp the element in an unwanted manner.
In another type of prior art transducer, a series of grooves are cut at right angles to each other not only in a first surface but additionally, in an opposite surface; however, the arrangement is potted in an epoxy for the purpose of widening the bandwidth of the transducer.